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Buildbot is a system to automate the compile/test cycle required by most software projects to validate code changes.
By automatically rebuilding and testing the tree each time something has changed, build problems are pinpointed quickly, before other developers are inconvenienced by the failure.
The guilty developer can be identified and harassed without human intervention.
By running the builds on a variety of platforms, developers who do not have the facilities to test their changes everywhere before checkin will at least know shortly afterwards whether they have broken the build or not.
Warning counts, lint checks, image size, compile time, and other build parameters can be tracked over time, are more visible, and are therefore easier to improve.

The overall goal is to reduce tree breakage and provide a platform to run tests or code-quality checks that are too annoying or pedantic for any human to waste their time with.
Developers get immediate (and potentially public) feedback about their changes, encouraging them to be more careful about testing before checkin.

The Buildbot was inspired by a similar project built for a development team writing a cross-platform embedded system.
The various components of the project were supposed to compile and run on several flavors of unix (linux, solaris, BSD), but individual developers had their own preferences and tended to stick to a single platform.
From time to time, incompatibilities would sneak in (some unix platforms want to use string.h, some prefer strings.h), and then the tree would compile for some developers but not others.
The buildbot was written to automate the human process of walking into the office, updating a tree, compiling (and discovering the breakage), finding the developer at fault, and complaining to them about the problem they had introduced.
With multiple platforms it was difficult for developers to do the right thing (compile their potential change on all platforms); the buildbot offered a way to help.

Another problem was when programmers would change the behavior of a library without warning its users, or change internal aspects that other code was (unfortunately) depending upon.
Adding unit tests to the codebase helps here: if an application’s unit tests pass despite changes in the libraries it uses, you can have more confidence that the library changes haven’t broken anything.
Many developers complained that the unit tests were inconvenient or took too long to run: having the buildbot run them reduces the developer’s workload to a minimum.

In general, having more visibility into the project is always good, and automation makes it easier for developers to do the right thing.
When everyone can see the status of the project, developers are encouraged to keep the tree in good working order.
Unit tests that aren’t run on a regular basis tend to suffer from bitrot just like code does: exercising them on a regular basis helps to keep them functioning and useful.

The current version of the Buildbot is additionally targeted at distributed free-software projects, where resources and platforms are only available when provided by interested volunteers.
The workers are designed to require an absolute minimum of configuration, reducing the effort a potential volunteer needs to expend to be able to contribute a new test environment to the project.
The goal is for anyone who wishes that a given project would run on their favorite platform should be able to offer that project a worker, running on that platform, where they can verify that their portability code works, and keeps working.

The Buildbot consists of a single buildmaster and one or more workers, connected in a star topology.
The buildmaster makes all decisions about what, when, and how to build.
It sends commands to be run on the workers, which simply execute the commands and return the results.
(certain steps involve more local decision making, where the overhead of sending a lot of commands back and forth would be inappropriate, but in general the buildmaster is responsible for everything).

The buildmaster is usually fed Changes by some sort of version control system (Change Sources), which may cause builds to be run.
As the builds are performed, various status messages are produced, which are then sent to any registered Reporters.

The buildmaster is configured and maintained by the buildmaster admin, who is generally the project team member responsible for build process issues.
Each worker is maintained by a worker admin, who do not need to be quite as involved.
Generally workers are run by anyone who has an interest in seeing the project work well on their favorite platform.

The workers are typically run on a variety of separate machines, at least one per platform of interest.
These machines connect to the buildmaster over a TCP connection to a publically-visible port.
As a result, the workers can live behind a NAT box or similar firewalls, as long as they can get to buildmaster.
The TCP connections are initiated by the worker and accepted by the buildmaster, but commands and results travel both ways within this connection.
The buildmaster is always in charge, so all commands travel exclusively from the buildmaster to the worker.

To perform builds, the workers must typically obtain source code from a CVS/SVN/etc repository.
Therefore they must also be able to reach the repository.
The buildmaster provides instructions for performing builds, but does not provide the source code itself.

Which create a Change object each time something is modified in the VC repository.
Most ChangeSources listen for messages from a hook script of some sort.
Some sources actively poll the repository on a regular basis.
All Changes are fed to the schedulers.

Schedulers

Which decide when builds should be performed.
They collect Changes into BuildRequests, which are then queued for delivery to Builders until a worker is available.

Builders

Which control exactly how each build is performed (with a series of BuildSteps, configured in a BuildFactory).
Each Build is run on a single worker.

Status plugins

Which deliver information about the build results through protocols like HTTP, mail, and IRC.

Each Builder is configured with a list of Workers that it will use for its builds.
These workers are expected to behave identically: the only reason to use multiple Workers for a single Builder is to provide a measure of load-balancing.

Within a single Worker, each Builder creates its own WorkerForBuilder instance.
These WorkerForBuilders operate independently from each other.
Each gets its own base directory to work in.
It is quite common to have many Builders sharing the same worker.
For example, there might be two workers: one for i386, and a second for PowerPC.
There may then be a pair of Builders that do a full compile/test run, one for each architecture, and a lone Builder that creates snapshot source tarballs if the full builders complete successfully.
The full builders would each run on a single worker, whereas the tarball creation step might run on either worker (since the platform doesn’t matter when creating source tarballs).
In this case, the mapping would look like:

and each Worker would have two WorkerForBuilders inside it, one for a full builder, and a second for the source-tarball builder.

Once a WorkerForBuilder is available, the Builder pulls one or more BuildRequests off its incoming queue.
(It may pull more than one if it determines that it can merge the requests together; for example, there may be multiple requests to build the current HEAD revision).
These requests are merged into a single Build instance, which includes the SourceStamp that describes what exact version of the source code should be used for the build.
The Build is then randomly assigned to a free WorkerForBuilder and the build begins.

The buildmaster maintains a central Status object, to which various status plugins are connected.
Through this Status object, a full hierarchy of build status objects can be obtained.

The configuration file controls which status plugins are active.
Each status plugin gets a reference to the top-level Status object.
From there they can request information on each Builder, Build, Step, and LogFile.
This query-on-demand interface is used by the html.Waterfall plugin to create the main status page each time a web browser hits the main URL.

The status plugins can also subscribe to hear about new Builds as they occur: this is used by the MailNotifier to create new email messages for each recently-completed Build.

The Status object records the status of old builds on disk in the buildmaster’s base directory.
This allows it to return information about historical builds.

There are also status objects that correspond to Schedulers and Workers.
These allow status plugins to report information about upcoming builds, and the online/offline status of each worker.

A developer commits some source code changes to the repository.
A hook script or commit trigger of some sort sends information about this change to the buildmaster through one of its configured Change Sources.
This notification might arrive via email, or over a network connection (either initiated by the buildmaster as it subscribes to changes, or by the commit trigger as it pushes Changes towards the buildmaster).
The Change contains information about who made the change, what files were modified, which revision contains the change, and any checkin comments.

The buildmaster distributes this change to all of its configured schedulers.
Any important changes cause the tree-stable-timer to be started, and the Change is added to a list of those that will go into a new Build.
When the timer expires, a Build is started on each of a set of configured Builders, all compiling/testing the same source code.
Unless configured otherwise, all Builds run in parallel on the various workers.

The Build consists of a series of Steps.
Each Step causes some number of commands to be invoked on the remote worker associated with that Builder.
The first step is almost always to perform a checkout of the appropriate revision from the same VC system that produced the Change.
The rest generally perform a compile and run unit tests.
As each Step runs, the worker reports back command output and return status to the buildmaster.

As the Build runs, status messages like “Build Started”, “Step Started”, “Build Finished”, etc, are published to a collection of Status Targets.
One of these targets is usually the HTML Waterfall display, which shows a chronological list of events, and summarizes the results of the most recent build at the top of each column.
Developers can periodically check this page to see how their changes have fared.
If they see red, they know that they’ve made a mistake and need to fix it.
If they see green, they know that they’ve done their duty and don’t need to worry about their change breaking anything.

If a MailNotifier status target is active, the completion of a build will cause email to be sent to any developers whose Changes were incorporated into this Build.
The MailNotifier can be configured to only send mail upon failing builds, or for builds which have just transitioned from passing to failing.
Other status targets can provide similar real-time notification via different communication channels, like IRC.